Recently, a type of image display apparatus which enlarges and projects an image displayed on LCD, LCOS (Liquid Crystal On Silicon) or DMD (Digital Mirror Device) onto a screen has been popular. The image display apparatus having the DMD can produce high brightness and high resolution images, but is too expensive for projection televisions and such commercial products. The image display apparatus having the LCD only produces low resolution images, and hardly provides high brightness due to its low aperture ratio. The image display apparatus having the LCOS, by contrast, is advantageously cheaper than the apparatus with the DMD, and can produce higher brightness and higher resolution images than the apparatus with the LCD.
In the image display apparatus having the LCOS, the light generated from a light source unit is separated into colors by a dichroic mirror. Each color light is adjusted to a predetermined polarization direction by a polarizer, and guided to the LCOS displaying an image corresponding to red, green or blue color. When reflected by the LCOS, each color light is converted into information light that carries the information of the image. The information light is transmitted through an analyzer, such as a polarizing beam splitter, and projected on a screen.
It is ideal if the light from the light source unit enters the LCOS at right angle. However, the light is actually a bundle of plural light rays, and contains skew rays that are inclined with respect to the display surface of the LCOS. These skew rays are reflected by the LCOS, and enter the polarizing beam splitter at an angle inclined from a prescribed incident axis. In this case, the polarizing beam splitter may transmit some light rays supposed to be reflected. The skew rays transmitted through the polarizing beam splitter will reduce the contrast of a projection image on the screen. At the same time, this contrast reduction due to the skew rays can be solved by a quarter-wave plate that compensates for the polarizing direction of the skew ray (see, for example, Japanese Patent Laid-open Publication No. 02-250026).
The quarter-wave plate is only effective to the skew rays inclined at 5° or below from a surface normal of the plate. Unfortunately, the skew rays generated in a projector are generally inclined at between 10° and 15° from the surface normal, and their polarizing direction cannot be compensated by the quarter-wave plate properly to provide sufficient contrast.
This angle dependency problem of the quarter-wave plate can be solved when a phase retarder, or so-called an O-plate, having a principal refractive index (main axis) inclined to the surface is adjusted in thickness and used as the quarter-wave plate. The O-plate used as the quarter-wave plate may be fabricated by cutting a crystal of uniaxial birefringent body, such as quartz, in a direction oblique to the main axis, or by applying and polymerizing rod-like liquid crystal molecules on a surface of the polarizing beam splitter (see, for example, “Wide Field of View Compensation Scheme for Cube Polarizing Beam Splitters” by M. G. Robinson et al, SID '03 Digest pp. 874, Society for Information Display).
Additionally, a polarization separating performance of the polarizing beam splitter can be enhanced and the image contrast reduction due to the skew ray can be prevented by forming a retardation layer (a phase difference layer) of two thin films with different refractive indices on a polarization separating layer of the polarizing beam splitter (see, for example, Japanese Patent Laid-open Publication No. 06-289222).
The image contrast reduction due to the skew ray can also be prevented by a retardation layer which is formed, as the quarter-wave plate, by obliquely depositing an inorganic dielectric material on a light valve (see, for example, Japanese Patent Laid-open Publication No. 10-206842).
However, the O-plate cut obliquely from the crystal may be impractical because it is difficult to shape and requires extra cost.
The O-plate of the rod-like liquid crystal molecules or such organic material, on the other hand, changes the inclination angle of the main axis depending on the property of the material. It is therefore difficult to set the main axis at an optically desirable inclination angle.
Additionally, the O-plate of the obliquely deposited inorganic dielectric material has the following drawback. To function as the quarter-wave plate, the O-plate needs to have main axis inclined at between 10° and 15° to the plate surface. In other words, the main axis of the O-plate should be inclined at between 75° and 80° to the surface normal of the plate. However, the actual O-plate fabricated by the oblique deposition has the main axis inclined at between 0° and 45° to the surface normal of the plate, and it is almost impossible to fabricate the O-plate having the main axis inclined at between 75° and 80°.
Even when the O-plate is fabricated by the oblique deposition or polymerization of the liquid crystal molecules, the O-plate needs to have a thickness of 1 μm or above to function as the quarter-wave plate. Such great thickness increases the haze, and the polarization is rather lowered.
Moreover, aforesaid retardation layer on the polarization separating layer will complicate the manufacturing process of the polarizing beam splitter.